The prior art is illustrated in particular by the following patents:
USP 3 791 963, FR 1 510 253, EP 70 657, EP 31 255 and DE 1 542 559.
The synthesis is fluoride media of this type of zeolite of MFI structure has already been disclosed in the French patent 2 567 868 and more recently in an article of J.L.GUTH and coll. (Proc. 7th Int. Zeolite Conf, Tokyo, August 1986, p. 121).
This synthesis comprises:
(a) a first step of forming a reaction medium comprising water, a silica source, an alumina source, a source of structurizing agent for supplying organic cations selected from the group consisting of tetrapropyl ammonium(TPA.sup.+) and tetrapropylphosphonium (TPP.sup.+) cations, this reaction medium further containing fluoride anions. The pH of the medium is generally lower than 10 and the molar ratios of the various constituents of the reaction medium are disclosed in French patent 2 567 868.
(b) a second step of heating the reaction medium formed in step (a) at a temperature ranging from about 80.degree. to 230.degree. C., preferably from 140.degree. to 210.degree. C., so as to obtain a crystallized solid which is separated,
(c) a third step of heating the solid obtained at the end of step (b) at a temperature higher than 400.degree. C., so as to remove, by decomposition and optionally by combustion if the treatment is performed in the presence of oxygen, the organic species supplied by the structurizing agent and contained in the solid after synthesis.
The reaction medium pH lower than 10 may be obtained either directly from one or more products forming the reaction medium, or by adding to said medium an acid, a base, an acid salt, a basic salt or a complementary buffer mixture.
Fluoride anions F.sup.- may be introduced into the reaction medium as fluorides, sodium fluoride NaF, ammonium fluoride NH.sub.4 F, acid ammonium fluoride NH.sub.4 HF.sub.2, tetraproprylammonium flouride (C.sub.3 H.sub.7).sub.4 NH, tetrapropylphosphonium fluoride (C.sub.3 H.sub.7).sub.4 PF, or hydrolyzable compounds capable of releasing fluoride anions in water, such as silicon fluoride SiF.sub.4 or sodium fluorisilicate Na.sub.2 SiF.sub.6.
Ammonium flouride or acid ammonium fluoride are preferred since they result in a zeolite of MFI structure, easy to convert to its protonic form without requiring ion exchange reactions.
Many silica sources can be used to form the reaction mixture, such as for example :
silicas as hydrogels, aerogels, colloidal suspensions,
silicas obtained by precipitation of soluble silicate solutions, or by hydrolysis of silicic esters such as tetraethyl ester of monoorthosilicic acid Si(OC.sub.2 H.sub.5).sub.4, or complexes such as sodium fluorosilicate Na.sub.2 SiF.sub.6 or ammonium fluorosilicate (NH.sub.4).sub.2 SiF.sub.6,
silicas prepared by extraction and activation of natural or synthetic crystallized compounds such as aluminum silicates, aluminosilicates, clays, etc... The silicas may be used in divided state or as agglomerates.
Examples of alumina sources are aluminum salts (sulfate, nitrate chloride, fluoride, acetate, for example), aluminum hydroxides and oxides, aluminates, esters such as tripropylester of monoorthoaluminic acid Al(OC.sub.3 H.sub.7).sub.3.
Instead of separate alumina and silica sources, combined sources of both oxides, such for example as amorphous silicaalumina gels, crystallized aluminosilicates, comprising clays and zeolites, can also be used.
The silica and alumina sources may be in soluble or solid form, but also in the form of agglomerates such as extrudates or pellets. Pellets are convenient for sources consisting essentially of raw zeolites or already agglomerated modified zeolites, which may be thus converted by the new process to preformed zeolites.
The sources of struturizing agent supplying organic cations are preferably tetrahydrocarbylammonium and tetrahydrocarbylphosphonium cations, whose hydrocarbyl group is advantageously an alkyl, preferably a propyl group.
Tetrapropylammonium (TPA.sup.+) or tetrapropylphosphonium (TPP+) cations, which are the preferred structurizing agents, are preferably added as salts, for example as bromides or fluorides, but they may also be generated in situ from tripropylamine or tripropylphosphine and a propyl halide.
The acids or acid salts, the bases or basic salts optionally added to bring the pH of the reaction medium to the desired value may be selected from usual acids, such as hydrofluoric acid HF, hydrochloric acid HCl, nitric acid HNO.sub.3, sulfuric acid H.sub.2 SO.sub.4, acetic acid CH.sub.3 COOH, or acid salts such as acid ammonium fluoride NH.sub.4 HF.sub.2, acid potassium fluoride KHF.sub.2, acid sodium sulfate NaHSO.sub.4, acid potassium sulfate KHSO.sub.4, acid sodium phosphate NaH.sub.2 PO.sub.4 and from usual bases, such as ammonia NH.sub.4 OH, sodium hydroxide NaOH, potassium hydroxide KOH, or usual basic salts such as sodium acid carbonate NaHCO.sub.3 or neutral carbonate Na.sub.2 CO.sub.3, sodium acetate CH.sub.3 COONa, sodium neutral or acid sulfides (Na.sub.2 S or NaHS) or buffer mixtures such as acetic acid CH.sub.3 COOH-sodium acetate CH.sub.3 COONa, ammonia NH.sub.4 OH-ammonium chloride NH.sub.4 Cl.
The morphology, the size and the kinetics of formation of zeolite crystals according to the process of the invention may be modified by introducing into the reaction medium complementary salts such as sodium chloride NaCL, potassium chloride KCl, sodium sulfate Na.sub.2 SO.sub.4 and/or crystals (crushed or not) or solid compounds related to the zeolites prepared by the process of the invention.
TABLE 1 ______________________________________ X-ray spectrum characteristics of zeolites of MFI structure according to the invention d.sub.hkl (.ANG.) I/I.sub.o d.sub.hkl (.ANG.) I/Io d.sub.hkl (.ANG.) I/Io ______________________________________ 11.08-11.26 VS 4.06-4.10 vl 2.772-2.793 vl 9.94-10.20 ml 3.99-4.05 l 2.725-2.749 vl 9.68-9.90 l 3.83-3.89 S 2.677-2.697 vl 8.98-9.08 vl 3.80-3.86 m 2.648-2.670 vl 8.00-8.09 vl 3.74-3.78 ml 2.605-2.619 vl 7.40-7.52 vl 3.70-3.74 ml 2.581-2.597 vl 7.03-7.22 vl 3.63-3.67 ml 2.545-2.557 vl 6.64-6.84 l 3.58-3.62 vl 2.508-2.526 vl 6.30-6.42 l 3.46-3.50 vl 2.479-2.501 vl 5.95-6.07 l 3.42-3.46 l 2.407-2.419 vl 5.67-5.79 l 3.38-3.42 vl 2.393-2.401 vl 5.54-5.61 l 3.33-3.37 l 2.326-2.340 vl 5.32-5.42 vl 3.29-3.33 vl 2.314-2.332 vl 5.10-5.23 vl 3.23-3.27 vl 2.195-2.209 vl 5.01-5.08 l 3.16-3.20 vl 2.104-2.120 vl 4.95-5.03 l 3.12- 3.16 vl 2.077-2.095 vl 4.84-4.93 vl 3.08-3.12 vl 2.070-2.084 vl 4.59-4.64 vl 3.03-3.07 l 2.004-2.022 l 4.44-4.50 vl 2.976-3.020 l 1.985-2.005 l 4.34-4.40 l 2.943-2.962 l 1.944-1.964 vl 4.23-4.29 l 2.855-2.881 vl 1.907-1.922 vl 1.866-1.881 vl ______________________________________ VS = very strong; S = strong; mS = middle to strong; m = middle; ml = middle to low; l = low; vl = very low
The solids obtained by the above-described synthesis procedure are zeolites of MFI structure whose X-ray diffraction diagrams have characteristics corresponding to the specifications of table 1. These zeolites of MFI structure approximately conform, after roasting, to the following chemical formula, expressed as oxides: EQU M.sub.2/n O, Al.sub.2 O.sub.3, xSiO.sub.2
wherein x may vary from 12 to 1,000 and M represents one or more compensation cation(s) of valence n. These solids must essentially contain, after the synthesis step and also after the step of removing organic compounds, fluorine. The zeolite fluorine content, determined by elemental analysis, is, for the roasted solids, i.e. those resulting from the above-described step (c), from 0.02 to 1.5% by weight, advantageously from 0.1 to 1.0 % and preferably from 0.2 to 0.8%.
The presence of fluorine in zeolites of MFI structure prepared according to the invention confers properties to these solids, particularly acid properties and ion exchange properties, completely different from those of zeolites of MFT structure synthesized in a conventional manner, i.e. in alkaline medium (for example USP 3 702 886). After synthesis and removal of the organic compound by roasting (steps a, b, c), the solids according to the invention are characterized by an infra-red vibration spectrum comprising, as shown in the figure for fluorine contents of 0.8% (curve 1) 0.2% (curve 2) and 0.05% (curve 3), bands conventionally attributed to Si-OH groups (3730 - 3750 cm.sup.-1 area) and to Al-OH structural groups (3580-3640 cm.sup.-1 area) of low intensity as compared with those of a zeolite of conventional MFI structure, having the same Si/Al ratio equal to 22 (curve 4, F %=0).
The absence or quasi absence of Al--OH structural groups in the zeolites according to the invention is confirmed by the ion exchange capacities of these solids. As a matter of fact the ion exchange capacities for cations such for example as Na.sup.+, K.sup.+, Ga.sup.3+, Pt(NH.sub.3).sub.4.sup.2+ etc... are much lower than the total theoretical ion exchange capacities, as calculated from the aluminum content of the crystalline structure.
These solids have no or only a few structural hydroxyl groups and a very reduced exchange capacity, yet surprisingly exhibti remarkable acid properties. Thus the ammonia thermodesorption used to estimate the overall acidity of a solid (number and strength of the different types of acid sites) show that solids including fluorine incorporated with the structure are very acid. The ammonia thermodesorption spectra are similar to those obtainable with conventional zeolites of MFI structure, but the acidity of the solids according to the invention is of a different nature.
Without being bound by any particular theory, it may be considered for example that these solids have, in place of at least a part of the conventional ##STR1## sites, sites of the following types: ##STR2##
The precise nature of the acid sites present in the solids according to the invention has still to be precisely stated, but it is clear that most of these sites are associated with the presence of fluorine and differ by their nature from acid sites of conventional MFT zeolites.
Fluorine introduction into zeolites has already been proposed for increasing the acidity of said solids (S.KOWALAK, React. Kinet. Catal. Lett, 27, 1985 p. 441 and J. Chem. Soc. Farad. Trans 1, 82, (1986), 2151; J. MIALE and C. CHANG USP 4,540,841). However, in the prior art, fluorine is introduced into the zeolite through modifications achieved after synthesis. Otherwise stated, a conventional synthesis, i.e. in alkaline medium, is first achieved, and then the solid is treated by a technique known as adapted to fix fluorine. These above-proposed techniques generally suffer from heavy defects. For example, as when treating the solid with fluorine gas, they are liable to result in a degradation of the crystalline order (USP 4,297,335). In the present catalyst preparation, fluorine is introduced into the zeolite during the synthesis and gives, on the contrary, very well crystallized solids.
By particular treatments it is possible to partially or completely remove fluorine from the solids involved in the composition of the catalysts according to the invention without modifyin their crystallinity. A technique for defluorinating the solids comprises treatment in ammonia solution within a temperature range from room temperature to 200.degree. C., for example (treatment in autoclave under autogenic pressure). The partial or complete fluorine removal has as a result:
the formation in the IR spectrum of two bands located about at 3740 and 3608 cm.sup.-1, corresponding, according to the scientific literature, respectively to ending silanol groups and structural Al-OH groups, and
the restoration of the ion exchange capacity, as determinable by the aluminum content of the solid structure.
Thus, depending on the defluorination treatment, solids containing a variable amount of Al--OH and Si--OH groups, and having a variable ion exchange capacity, can be obtained for the same Si/Al ratio of structure. A partially defluorinated solid hence contains, in addition to conventional acid sites of Al-OH type, which may act as exchange sites, particular acid sites, whose exact nature is still not completely known, but which undenisably result from the fluorine introduction in the solids during the synthesis.
By taking advantage of this particularity of the solids it has been possible to prepare bifunctional catalysts containing at least one group VIII metal selected from platinum and palladium and for example adapted for isomerization of a C.sub.8 aromatic hydrocarbon, more generally of a C.sub.8 aromatic cut, and whose acid properties are of a new type.